This is a prerequisite for properly constructed framework bundles.
On certain Apple platforms (iOS, tvOS, watchOS), bundles are used
in "shallow" format, meaning that the directory structures are
flattened compared to the one used in macOS bundles.
shallow_bundle allows the difference to be expressed independently
of the platform. Note that the term "shallow bundle" is used by
Apple in Xcode internals.
Change-Id: I1189c52b0ea66843c313783176c11cc2af97ad25
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>